Method for enhancing the weaving of a warp yarn fabric having a high modulus of elasticity

ABSTRACT

A method for enhancing the weaving of a warp yarn fabric having a high modulus of elasticity in which a sheet of parallel warp yarns is continuously unwound from a beam. The sheet is fed over a whip roll and a shed is formed using healds, the shed being defined in the forward direction of the warp by a shed opening point on an inlet side and a fell point on an opposite side. A weft yarn is inserted into the shed to form a resulting fabric which is evenly pulled and wound. The warp yarns are sufficiently heated in an area adjacent to the shed opening point to cause a local reduction in the modulus of elasticity of the warp yarns. Subsequently, the warp yarns are cooled in the shed before reaching the healds.

FIELD OF THE INVENTION

The invention relates to the field of textile weaving and, morespecifically, to a method making it possible to improve the weaving of afabric of warp yarns having a high modulus of elasticity.

BACKGROUND OF THE INVENTION

As is known, in a conventional weaving loom, the warp yarns are unwoundfrom a beam and then pass over a back-rest roller. Downstream of thelatter, depending on the selected weave, the various parallel warp yarnsare drawn alternately upwards and downwards in order to form the shed.The various warp yarns subsequently meet again at the fell point, wherethe comb beats up the cloth after each weft insertion.

Within the shed, the yarns are drawn upwards and downwards by means ofhealds. It will easily be appreciated that these warp yarns drawn inthis way undergo mechanical stresses whenever the shed opens. Thesestresses lay the yarns very firmly onto their successive guides (yarnguides, shield eyes, comb, etc.), thus causing fraying and bringingabout yarn breaks which bring the weaving loom to a stop, thus slowingthe actual production speed and impairing the uniformity of the clothobtained.

Some designers of weaving looms attempt to reduce the stresses impartedto the yarns by giving the back-rest roller a reciprocating movement insynchronism with the opening of the shed. Unfortunately, theconsiderable weight of the back-rest roller and, consequently, its highinertia reduces mobility, the more so because the beating-up frequencyis high.

It will be appreciated that the mechanical stresses which the yarnsundergo are the greater the higher the beating-up speeds. Thus, it wasfound that, for high-speed weaving looms, that is to say mainly those inwhich the weft yarn is driven by water jet or air jet, it is difficultto exceed the speed of 1000 to 1200 beats per minute, without seeingsigns of serious damage to the warp yarns and a large number of breaks,the more so because the warp yarns have a high modulus of elasticity.

This phenomenon, which is the more pronounced, the more rigid the warpyarns are, gives rise, in particular, to more frequent malfunctionsduring the weaving of polyester than during the weaving of polyarnide.

The invention therefore attempts to solve the problem of warp yarnbreaks following high mechanical stresses experienced by the yarns atthe moment the shed opens and, as a consequence, to increase thebeating-up speed of the looms, whilst at the same time maintaining thesame weaving quality.

SUMMARY OF THE INVENTION

The invention relates to a method for improving the weaving of a fabriccomprising warp yarns having a high modulus of elasticity, in which:

a layer of parallel warp yarns is unwound continuously from a beam;

the said layer is passed over a back-rest roller;

a shed is formed by means of healds, the said shed being defined, in thedirection of advance of the warp, by a shed opening point at theentrance and by a fell point on the other side;

a weft yarn is inserted into the shed in the vicinity of the fell pointin order to form a fabric;

and finally, the fabric thus formed is pulled and wound uniformly.

This method is characterized in that, in the vicinity of the openingpoint of the shed, the warp yarns are heated to a temperature sufficientto bring about a local reduction in the modulus of elasticity of thewarp yarns, followed by cooling of the warp yarns in the shed beforethey reach the healds.

As defined herein modulus of elasticity (or Young's modulus) means theleading coefficient (or the slope of the origin) of the curve whichgives, on the ordinate, the pulling force and, on the abscissa, theelongation which this force causes. This modulus is expressed innewtons/tex or in gigapascals (GPa). As an example, the modulus ofelasticity of cold polyester yarns is of the order of 10 GPa for textileyarns having high residual elongation or elongation at break (greaterthan 35%). It may reach 15 GPa for high-strength yarns with low residualelongation (of the order of 15%). By way of comparison, the Young'smodulus of polyarnide yarns is of the order of 5 GPa. In both cases,heating beyond the second-order transition temperature causes aconsiderable reduction in the modulus of elasticity when the yarn isthen stretched. Typically, the modulus of elasticity under hotconditions may decrease, and, in some cases, reaches one tenth of themodulus under cold conditions.

In other words, the invention involves heating the warp yarns at theopening point of the shed, so as to reduce their modulus of elasticityin order to make them more flexible and, more specifically, in order toallow them, in the region of entry into the shed, to absorb the pulsedincreases in tension which are attributable to the movements of thehealds.

In general terms, at low speeds of advance, cooling takes place bynatural convection, but the invention also embraces the variants inwhich additional means of any kind are used to make cooling easier.

Furthermore, if the warp yarns are thermoplastic and therefore have asoftening temperature (TR) and a melting point (TF), the warp yarns areadvantageously heated to a temperature between the softening temperature(TR) and the melting point (TF) of the warp yarns. Thus, in some cases,heating the warp yarns makes it possible to cause stretching, while atthe same time limiting the tension to which these yarns are subjectedduring the opening of the shed, thereby reducing the risks of breakage.

Thus, according to the invention, these warp yarns are stretched in theheated zone as a result of the intermittent pull imparted to them by themovement of the healds, said pull being combined with the uniformtake-up of the winding of the cloth. Surprisingly, the intermittentstretching of the yarns which is carried out in this way does not, ascould have been foreseen, give rise to irregularities in the warp yarnsand therefore in the cloth. Quite to the contrary, an improvement isfound in the uniformity of the yarns processed in this way, in terms ofboth linear density and dyeing affinity.

In practice, heating takes place either by convection or by radiation orby contact.

In a practical embodiment, heating is carried out by contact, by meansof a heating pad protected from abrasion by a chromium-plated or ceramiccovering. In other words, before entering the shed, or just on enteringthe latter, the warp yarns burst by sliding on a heating element, ofwhich the length, counted in the direction of passage of the yarn, iscalculated in order to cause the desired increase in temperature withinthe yarns, this being as a function of the speed of advance and of thesize of the yarns.

Advantageously, in practice, the pad is combined with means making itpossible to move the yarn layer away when the weaving loom stops. Thus,as soon as the warp yarns have stopped, the heating pad is moved awayfrom the warp yarns in order to avoid causing damage to these, or evenmelting them in an extreme case.

Advantageously, the heating pad is located above the warp in order toavoid residual heating by convection when the pad is being moved away.

In a preferred embodiment, the device for moving away the heating padoperates by default, that is to say it moves the pad away automaticallyas soon as the advance of the yarn stops, whether after a downstreamyarn break or after a failure in the supply of electricity or compressedair to the various members of the installation.

In a preferred embodiment, the heating pad is arranged in a zonecontained between the back-rest roller and leasing guides arranged inthe vicinity of the back-rest roller and defining the opening points ofthe shed. Thus, heating is obtained uniformly for all the yarns broughttogether in a plane zone where they are all parallel.

This method makes it possible to process a large number of chemicalyarns, for example polyarnides, polyester and, advantageously, partiallyoriented yarns, commonly called "POYs", such as, in particular, thosedescribed in the Patents U.S. Pat. Nos 3,771,162 and 3,772,872. In fact,the weaving method according to the invention makes it possible toachieve stretching of the yarn and thus executes, in a single step, thetwo operations of characteristic intermittent stretching and weaving.Previously, and, in order to obtain a POY-based cloth, conventionaluniform drawing has been carried out beforehand, followed, completelyindependently, by weaving.

It also makes it possible to process yarns for industrial use, such as,for example, glass or steel yarns.

BRIEF DESCRIPTION OF THE DRAWINGS

The way in which the invention is implemented and the advantages whicharise from it will emerge clearly from the description of the followingfigure. FIG. 1 is the circuit along which the warp yarns run within aweaving loom in accordance with a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As already stated, the invention relates to a method for improving theweaving of warp yarns having a high modulus of elasticity. This methodmay be employed on a conventional weaving loom, to which are addedelements making it possible to carry out the characteristic function ofthe method.

Thus, the conventional weaving loom has, in the direction of run of theyarn, a beam (1) which is mounted on a shaft (2) and on which all theparallel warp yarns are wound. These warp yarns (3) are unwound from thebeam (1) as far as a back-rest roller (4), from which they assume asubstantially horizontal direction. In a particular embodiment, thesewarp yarns (3) subsequently pass between two leasing guides (5, 6).Beyond these leasing guides (5, 6), they are taken up by healds (7, 8),the purpose of which is to move the various warp yarns (3) upwards ordownwards in order to form the shed (9) and make it possible to insertthe weft yarn (16). After the healds (7, 8), the warp yarns meet againat the fell point (10), onto which the comb (11) beats up after eachreopening of the shed. Beyond the fell point, the cloth (12) thus formedpasses by way of various transfer rollers (13) in order to end up at thewinding system (14).

As already stated, the invention involves heating the warp yarns in thevicinity of the opening point of the shed, in order to reduce theirmodulus of elasticity and, if appropriate, allow them, as a result ofthe action of the loom, to undergo intermittent stretching just beforeactual weaving. As already stated, the intermittent stretching resultsin a uniformity of the cloth, which is an unexpected effect and explainsthe fact that it has not been sought hitherto.

Thus, various means may be suitable for ensuring this heating. In theembodiment illustrated, there is a heating pad (20) which is presentover the entire width of all the warp yarns and which can come intocontact with the layer of warp yarns between the back-rest roller (4)and the leasing guides (5, 6). More specifically, there is a transverseheating element, the lower face of which is covered with a materialhaving very high surface hardness and a low coefficient of friction, inorder to prevent any abrasion by the warp yarns which would causesubsequent damage to the warp yarns themselves. There may be, inparticular, an outer chromium-plated or ceramic coat (24).

Of course, the invention embraces all types of heating of the pad,particularly those employing electrical energy, by means of suitableconnections (22).

As already stated, the pad (20) may be combined with means (21) makingit possible to position the pad (20) in contact with the warp yarns (3)and, above all, to withdraw this pad if the machine stops, the purposeof this being to prevent the yarns from being damaged, or even melted inan extreme case. These may be purely mechanical means or thosepreferably acting by means of hydraulic jacks, or even electromechanicalsystems. Preferably, the position of rest of the pad is away from theyarn layer, in such a way that, in the event of a breakdown of themoving-away means, the pad does not remain in contact with the yarns,but, on the contrary, automatically moves away from these.

Preferably, the pad is positioned above the yarn layer.

Of course, the invention is not limited to the embodiment illustrated,in which heating is carried out upstream of the leasing guides (5, 6),but also embraces the variants in which heating takes place in thevicinity of or just after these.

Moreover, the method may be carried out with the aid of a heating pad,of which the contact zone with the yarns is a few centimeters, but alsowith the aid of means operating by radiation or by convection, in whichthe heating element does not come directly into contact with the warpyarns.

As already stated, in the particular case of thermoplastic yarns, theinvention involves raising the temperature of the yarn beyond itstransition temperature, but below the melting point. Thus, with regardto the processing of polyarnide (6--6), it was seen that the result wasuseful if the temperature was of the order of 180° C. to 200° C. Asregards polyarnide 6, this temperature is 170° C. to 190° C. Wherepolyester is concerned, this temperature is in the neighborhood of 200°C. to 220° C.

Serimetric tests were conducted on unwoven warp yarns from a fabricproduced, on the one hand, according to the method and, on the otherhand, without the characteristic heating of the invention being carriedout. Thus, it is seen that the strength of the processed yarns accordingto the invention is markedly increased, typically by 10% for alreadystretched yarns and by more than 100% for POYs. Additionally, excellentuniformity of the yarns in terms of both linear density and dyeingaffinity is found.

Furthermore, elongation at break is reduced and may, for example, gofrom 60 to 40% for yarns previously stretched. In the particular case ofa POY, the elongation at break (or residual elongation) may go from 400%before stretching to 50% after stretching. Finally, the spread relatingto this elongation at break is greatly reduced, by a factor of 5, thuscorresponding to homogenization of the qualities of the warp yarns. Thishomogenization is also seen in terms of dyeing affinity, the uniformityof which is improved.

It may be gathered from the foregoing that the method according to theinvention proves advantageous on different levels. Thus, the processingof the yarn during weaving makes it possible to increase the operatingspeed by more than 20%, that is to say, typically, making it possible togo from 1000 beats per minute to more than 1200. Furthermore,weaveability, that is to say performance, is increased and typicallygoes from 95% to 97% or even 98%, this increase in performance being allthe more appreciable because it is combined with the fact that thebeating-up speed is likewise increased.

What is claimed is:
 1. A method for improving the weaving of a fabric ofwarp yarns having a high modulus of elasticity, said method includingthe steps of:unwinding a layer of parallel warp yarns continuously froma beam; processing said layer over a back-rest roller; forming a shed bymeans of healds, said shed being defined, in the direction of advance ofthe warp, by a shed opening point at the entrance thereof and by a fellpoint on an opposite side; inserting a weft yarn into the shed in thevicinity of the fell point in order to form a fabric; heating the warpyarns in the vicinity of the opening point of the shed to a temperaturesufficient to cause a local reduction in the modulus of elasticity ofsaid warp yarns; cooling said warp yarns in the shed following saidheating step before said yarns reach the healds; and pulling and windingthe formed fabric uniformly.
 2. A method as recited in claim 1, whereinthe warp yarns have a softening temperature and a melting point, whereinthe method includes the steps of:heating the warp yarns during saidheating step to a temperature between the softening temperature and themelting point of said yarns; and stretching the warp yarns byintermittent pulling in the heated zone.
 3. A method as recited in claim2, wherein the warp yarns are partially oriented yarns (poys).
 4. Amethod as recited in claim 1, wherein said heating step takes place byone of the group consisting of convection, radiation and contact.
 5. Amethod as recited in claim 1, wherein said heating step includes thestep of heating by contacting said warp yarns using a heating padprotected against abrasion by a covering which is one of ceramic andchromium.
 6. A method as recited in claim 5, including the step oflocating the heating pad above the yarn layer prior to heating.
 7. Amethod as recited in claim 5, including the step of combining saidheating pad with means enabling the yarn layer to move away from saidpad when the beam stops.
 8. A method as recited in claim 7, includingthe step of moving the yarn layer relative to said heating pad when theyarn layer is at rest.
 9. A method as recited in claim 5, including thestep of arranging said heating pad in a zone contained between theback-rest roller and leasing guides which are arranged in the vicinityof the backrest roller for defining the shed opening point.